Expired questioncommonplace,to be

Custom has not commonly been regarded as a subject of any great moment. The inner workings of our own brains we feel to be uniquely worthy of investigation, but custom, we have a way of thinking, is behaviour at its most commonplace.

我的问题是为什么这段话里不能说 custom is at its most commonplace behaviour而且为什么用to be不用is， 谢谢 ：Ｄ